Various types of batteries have been developed heretofore, and in every battery, a packaging material is an essential member for sealing battery elements such as an electrode and an electrolyte. Metallic packaging materials have been often used heretofore for battery packaging.
On the other hand, in recent years, batteries have been required to be diversified in shape and to be thinned and lightened with improvement of performance of electric cars, hybrid electric cars, personal computers, cameras, mobile phones and so on. However, metallic battery packaging materials that have often been heretofore used have the disadvantage that it is difficult to keep up with diversification in shape, and there is a limit on weight reduction.
Thus, in recent years, there has been proposed a film-shaped laminate with a base material, an adhesive layer, a metal layer and a sealant layer laminated in this order as a battery packaging material which is easily processed into diversified shapes and is capable of achieving thickness reduction and weight reduction (see Patent Document 2). In a process for producing a battery using such a battery packaging material, a label showing battery product information etc. may be attached to a surface of the battery packaging material, or instead of using a label, product information etc. may be directly printed on a surface of the battery packaging material with ink jets.
However, there is the problem that if by means of the label or printing, a discrimination mark identical to that of a regular maker is attached to a battery forged by a maker different from the regular maker, it is difficult to distinguish between the regular product and the forged product.
As a method for solving such a problem, for example, Patent Document 1 discloses an electrochemical cell packaging material including a multilayer film having a structure in which a base material layer, an adhesive agent layer, a metal foil layer and a heat-bondable resin layer are laminated in this order, the base material layer or adhesive agent layer containing a pigment as a discrimination mark. According to the method described in Patent Document 1, an electrochemical cell packaging material having a discrimination mark is obtained which is discriminable from the outside and hard to be forged.
A film-shaped battery packaging material as in Patent Document 2 is formed in such a manner that a battery element can be sealed by heat-welding the peripheral edge of the sealant layers by heat sealing with the sealant layers facing each other.
Nowadays, in view of prevention of forgery, etc., it may be required to impart designability, discriminability or the like to the external appearance of a battery, and decorativeness may be imparted by blending a pigment etc. in a layer situated on the outermost layer of a battery packaging material. For example, Patent Document 3 discloses a lithium ion secondary battery outer packaging material in which at least a first adhesive layer, a metal foil layer, a second adhesive layer and a sealant layer are laminated in this order on one surface of a base material layer, and the other surface of the base material layer is provided with a discrimination layer that is a resin layer with a glossiness of 3.0 or less for discrimination, the discrimination layer containing a pigment for discrimination, the pigment including particles covered with a resin, the pigment having a volume resistivity of 5×10−2 Ω·m or more.